There is a conventional technology in which, in an inverter that applies a high-frequency voltage to a compressor to heat the compressor, when the voltage vector is changed from a zero vector V0 or a zero vector V7 to a real vector V3 or a real vector V4, switching elements are controlled such that only switching elements for one phase are turned off, thereby reducing the off time of the switching elements and thus suppressing a drop in the output voltage (for example, Patent Literature 1).